I have hyline browns - they are a Rhode Island Red Cross.  Large good egg layers.  My other three chickens are happy and hale but Half Beak, the one who died, died from heart-stopping pain.  Brought on by issues with her egg-laying process, don't know detail but do know she was in an insane amount of pain, laid a shell-less egg was fine then a couple of days later, ie two eggs were laid fine and then came another malfunction she died on the spot.  We (me and the other chickens) were sad to lose her but also glad she wasn't in that much pain any more.  I have since heard that these egg-laying malfunctions happen quite often.  So that is another option if they are a developed layer breed.

I don't know anything about diseases.  It's possible there's an environmental factor if you mean your sister only had them for a week but if she's had them for a while and then they all died in a week then it would be something that's changed in that week.  And if they're old and from the same clutch it's possible they've died of natural causes.  

I dunno Tommo, all dying in a week from no obvious cause, it is a bit mystifying isn't it.  I haven't heard anything about rats causing disease in chickens but it seems to me like a reasonable possibility.